In the prior art, a phase shifter is employed as one of the component circuits of a phased array antenna and the like. FIG. 13 is a block diagram schematically showing the configuration of a prior art phased array antenna. In the figure, a phased array antenna 200 includes a plurality of antenna elements 211, 212, 213, and 214. The antenna 200 changes the direction D of an incoming or outgoing electromagnetic wave by controlling the phase of the electromagnetic waves in the antenna elements 211, 212, 213, and 214.
The antenna 200 includes amplifiers 221, 222, 223, and 224, all of which amplify microwaves going out from or coming into the corresponding antenna elements 211, 212, 213, and 214, and phase circuits 231, 232, 233, and 234, all of which shift the phases of microwaves going out from or coming into the corresponding antenna elements 211, 212, 213, and 214. The phase circuits 231, 232, 233, and 234 are connected to a signal source 260 and a signal receiver 270 via corresponding directional couplers 251, 252, 253, and 254.
The antenna 200 also includes a control circuit 240 which controls the phase circuits and the directional couplers. More specifically, the control circuit 240 controls the phase shift of the phase circuits 231, 232, 233, and 234 with 5-bit control signals Pc1, Pc2, Pc3, and Pc4, respectively, and switches the connection of each phase circuit to the signal source 260 or to the signal receiver 270 with a control signal Kc.
FIG. 13 shows a phased array antenna which has four antenna elements for simplicity of description. There are more than four antenna elements in an actual phased array antenna.
FIG. 14 shows the specific configuration of the phase circuit having input and output terminals 23a and 23b, respectively. As shown in FIG. 14, each of the phase circuits 231, 232, 233, and 234 of FIG. 13 comprises five switched-line phase shifters 230a, 230b, 230c, 230d, and 230e, all of which provide different phase shifts. The phase shift is defined as the difference in phase between signals at the phase shifter output and input.
The first shifter 230a comprises first and second transmission lines 13 and 14 which have electrical lengths that differ by .lambda./32 (.lambda. is the wavelength of a propagating microwave), an input switch 311 which selects between input terminals of the transmission lines 13 and 14, and an output switch 312 which selects between the output terminals of the transmission lines. The second to fifth shifters 230b, 230c, 230d, and 230e have almost the same configuration as that of the first shifter 230a. However, in the second shifter 230b, the difference in electrical length between the first and second transmission lines is .lambda./16; in the third shifter 230c the difference in electrical length between the first and second transmission lines is .lambda./8; in the fourth shifter 230d the difference in electrical length between the first and second transmission lines is .lambda./4; and in the fifth shifter 230e the difference in electrical length between the first and second transmission lines is .lambda./2.
Further, the switching actions of each pair of the input and output switches 311 and 312 of each phase shifter are respectively controlled by switch control signals Pca, Pcb, Pcc, Pcd, and Pce as a control signal of each phase circuit. In the phase circuit so constructed, the phase of microwave input can be varied in steps of 11.25.degree. in the range of from 11.25.degree. to 348.75.degree. using a 5-bit control signal.
FIG. 11(a) shows a detailed configuration of the above-mentioned switched-line shifter. Note that for the simplicity of description a phase shifter 230 represents the shifters 230a, 230b, 230c, 230d, and 230e without distinction in FIG. 11(a) because the distinction among these shifters is only the phase shift as described with respect to FIG. 14.
The phase shifter 230 includes an input switch 311 (shown in FIG. 14) comprising a first input-side FET element 50a connected between a high-frequency frequency input terminal (RF input terminal) 2 and the input terminal of a transmission line 13 and a second input-side FET element 50c connected between the RF input terminal 2 and the input terminal of a second transmission line 14. The phase shifter 230 also includes an output switch 312 (shown in FIG. 14) comprising a first output-side FET element 50b connected between a high-frequency output terminal(RF output terminal) 3 and the output terminal of the transmission line 13 and a second output-side FET element 50d connected between the RF input terminal 3 and the output terminal of the second transmission line 14.
Since all elements constituting the phase shifter 230 are fabricated on a monolithic microwave IC (MMIC) substrate, GaAs MESFETs are employed as the FET switches 50a, 50b, 50c, and 50d. The bias voltage applied to the sources and drains of the FET switches 50a, 50b, 50c, and 50d is provided by the power-source voltage (5V) because of restrictions on the power-source from the external system (a phased array antenna control circuit). In each of the FET elements fabricated on the substrate there is an effective inductance L between the source and the drain. Therefore, as shown in FIG. 11(a), the power-source voltage (5V) is applied via a bias resistance 1 to the source of one of the four FET switches constituting the phase shifter (here the one is the FET switch 50d), thereby setting the potential between the source and drain of each FET switch at 5V.
In the prior art phase shifter 230, 5V is applied to the gate terminals 5a, 5b, 5c, and 5d of the respective FET switches as the on-potential which places each FET in the on-state, and 0V is applied to the gate terminals 5a, 5b, 5c, and 5d of the respective FET switches as the off-potential which places each FET element in the off-state.
In the phased array antenna 200 shown in FIG. 13, a microwave signal generated by the signal source 260 is supplied to the phase circuits 231, 232, 233, and 234 via the directional couplers 251, 252, 253, and 254, respectively. The microwave signal is then subjected to a process that shifts its phase forward or backward by a given amount in each of the phase circuits before being applied to the amplifiers 221, 222, 223, and 224, respectively. Finally, the microwave signals amplified in the amplifiers are radiated from the antenna elements 211, 212, 213, and 214 into the air.
The traveling direction of the microwaves radiated by the antenna 200 is a direction D that is perpendicular to a wavefront W. The wavefront W consists of parts having the same phase in the microwave signals radiated from the antenna elements. In other words, microwaves are radiated from the antenna 200 in the direction D. The radiation direction D depends on the phase shift set by the control signals Pc1, Pc2, Pc3, and Pc4 in the phase circuits 231, 232, 233, and 234.
The only electromagnetic waves coming into the phased array antenna 200 are electromagnetic waves reflected from a target, i.e., from the direction D. The incoming electromagnetic waves are supplied to the amplifiers 221, 222, 223, and 224 via the antennas 211, 212, 213, and 214. The incoming electromagnetic waves are amplified in the amplifiers and then supplied to the receiver 270 via the directional couplers 251, 252, 253, and 254.
As described with respect to FIG. 14, the phase shifters 230a, 230b, 230c, 230d, and 230e, each of which is an element of the phase circuits 231, 232, 233, and 234, respectively, have a phase shift controlled by the control signals Pc1, Pc2, Pc3, and Pc4 from the control circuit 240. Thereby, the direction D of outgoing and incoming electromagnetic waves from the phased array antenna 200 vary within a given range. Consequently, the phased array antenna 200 can scan using the microwave-sending-and-receiving operation described above.
Next, an explanation is given of the operation of the phase circuit and the phase shifter, a component of the phase circuit.
In the phase circuits 231, 232, 233, and 234, the phase shifts produced by the components, i.e., phase shifters 230a, 230b, 230c, and 230e, are controlled by the control circuit 240 shown in FIG. 13. In this case, when an RF signal is input to the input terminal 23a of each of the phase circuits 231, 232, 233, and 234, the phase of the RF signal successively changes by a given amount in each phase shifter of each phase circuit. As a result, in each phase circuit, the phase of the RF signal changes by the total of the phase amounts set in the first to fifth phase shifters.
The setting of the phase amount of the phase shifter 230, i.e., the switching of the phase amount, is carried out by the switching of the input-side switch 311 or the output-side switch 312.
As more specifically discussed with respect to FIG. 11(a), the RF signal input from the RF input terminal 2 is transmitted through either the first transmission line 13 or the second transmission line 14 of the switched-line phase shifter 230 and is output from the RF output terminal 3. In this case, for example, 5V is applied to the gate terminal 5a of the first input-side FET switch 50a and the gate terminal 5b of the first output-side FET switch 50b, while 0V is applied to the gate terminal 5c of the second input-side FET switch 50c and the gate terminal 5d of the second output-side FET switch 50d, when the RF signal is transmitted through the first transmission line 13. On the other hand, 0V is applied to the gate terminal 5a of the first input-side FET switch 50a and the gate terminal 5b of the first output-side FET switch 50b, while 5V is applied to the gate terminal 5c of the second input-side FET switch 50c and the gate terminal 5d of the second output-side FET switch 50d, when the RF signal is transmitted through the first transmission line 14. Accordingly, by switching the voltages applied to the gate terminals 5a, 5c, 5b, and 5d of the first and second input-side FET switches and the first and second output-side FET switches, respectively, the phase of the input RF signal can be changed.
As described above, in the prior art phased array antenna 200, the component phase shifters use the power-source voltage of the system (the control circuit of the array antenna) as the control voltage. That is, there are applied the on-potential 5V and the off-potential 0V to the bias terminals of the phase shifters, i.e., the gate terminals 5a, 5b, 5c, and 5d of the FET switches of the phase shifter. In this case, the bias voltage (off-potential) which places the FET switches in the off-state is much lower (0V) than the pinch-off potential which cuts off the operating current between the source and the drain. The pinch-off potential is generally 4V which is 1V lower than the voltage applied to the source and the drain. For this reason, at the time of the phase switching of the phase shifter, i.e., the on-off switching of the first and second FET switches of the input-side and output-side switches 311 and 312, respectively, the phase change of the phase shifter output is delayed.
FIG. 12(a) shows the phase change of the output of the phase shifter shown in FIG. 11(a). In the figure, L13 indicates the phase of the output of the phase shifter when the FET switches 50a, 50b, 50c, and 50d are controlled so that the microwave input is transmitted through the first transmission line 13; L14 indicates the phase of the output of the phase shifter when the FET switches 50a, 50b, 50c, and 50d are controlled so that the microwave input is transmitted through the second transmission line 14; t1 and t2 indicate the phase-switching times at which the transmission lines are switched in the phase shifter. The line Ph0 shows the ideal phase change of the output of the phase shifter, and the line Ph1 shows the actual phase change. As will be apparent from FIG. 12(a), in the real phase shifter, it takes time from the phase-switching time for the output to reach the desired phase.
FIG. 12(b) shows the change of the output power of the phase shifter at the phase-switching times. Line Pout0 of FIG. 12(b) indicates the ideal change of the output power of the phase shifter from the phase-switching time t1 to t2, and the line Pout1 indicates the actual change. As will be observed from the figure, the rise of the output power has a delay at the phase-switching times of the output of the phase shifter.
As described above, in the prior art phase shifter, the rise of the output power is delayed at the phase-switching times of the output of the phase shifter, partly because of capacitances between the gate and source and between the gate and drain of the component FET switch of the phase shifter.
In addition to the rise time problem in the output of the phase shifter at the phase-switching times, the pinch-off voltages of the FETs are not uniform among different lots or on the same wafer. Therefore, the delays of the phase shifters are not uniform.
FIG. 11(b) shows a configuration of the measurement circuit for measuring the change of the output voltage of the phase shifter shown in FIG. 12(b). In this measurement circuit, a switch circuit 300a is connected to the output terminal 3 of the phase shifter 230, and includes an input node 31 and first and second output nodes 32a and 32b. In the switch circuit 300a, an input node 31 is connected to the output terminal 3, the first output node 32a is connected to a measurement terminal 301 and the second output node 32b is connected to the ground. The switch circuit 300a is controlled by the control signal Pc of the FET switch of the phase shifter 230. Specifically, in the switch circuit 300a, the input node 31 is connected to the output node 32a when microwaves are transmitted through the second transmission line 14 in the phase shifter, and the input node 31 is connected to the output node 32b when microwaves are transmitted through the first transmission line 13 in the phase shifter shown in FIG. 11(a). Therefore, at the measurement terminal 301, there appear the microwaves which have been transmitted through the second transmission line 14.